Chain polymerization of unsaturated monomers can be divided into free radical, ionic, and coordination polymerizations. Ionic polymerization includes anionic and cationic polymerizations. Cationic polymerization is usually initiated by the Lewis acids such as BF3. Polyisobutylene rubber is the commercially important polymer made by the cationic polymerization. Anionic polymerization is usually initiated by alkyllithiums such as n-BuLi. Many anionic polymerizations are devoid of any termination reaction, and they are thus called “living” polymerization. Living anionic polymerization has led to the creation of thermoplastic elastomers such as SBS (styrene-butanediene-styrene block copolymers).
Coordination polymerization includes the Ziegler-Natta polymerization and the metallocene or single-site polymerization. The Ziegler-Natta polymerization is performed with zirconium or titanium salts, such as TiCl4, ZrCl4, and VCl4, as catalysts and alkyl aluminum compounds, such as trimethyl aluminum, as cocatalysts. Metallocene catalyst was discovered by Kaminsky in the early 1980's (see U.S. Pat. Nos. 4,404,344 and 4,431,788). Metallocene catalyst comprises a transition metal complex that has one or more cyclopentadienyl (Cp) ligands. Unlike the Ziegler-Natta catalysts which have multiple active sites of polymerization, metallocene catalysts have only “single” polymerization site, and therefore they are called “single-site” catalysts. Many non-metallocene single-site catalysts have also been developed over the past decade.
Among the chain polymerizations, free radical polymerization is the most widely used in the polymer industry. Commonly used free radical initiators include peroxides, azo compounds, and persulfates. Unlike ionic initiators or coordination catalysts which require restricted conditions such as moisture and impurity free reaction systems, free radical polymerization can readily tolerate moisture and impurities. More importantly, free radical polymerization can tolerate functional monomers such as hydroxyl, carboxyl, and amino monomers. Thus, free radical polymerizations are exclusively used for making hydroxyl acrylic resins, polyacrylic acid, olefin-acrylic copolymers, and many other functional polymers.
Since the late 1990s, olefin polymerization catalysts that incorporate late transition metals (especially iron, nickel, or cobalt) and bulky α-diimine ligands (or “bis(imines)”) have been investigated. These late transition metal catalysts (LTMC) are of interest because, unlike the early transition metal metallocenes or Ziegler catalysts, the LTMC can incorporate alkyl acrylate comonomers into polyolefins. See U.S. Pat. Nos. 5,866,663 and 5,955,555.
However, the LTMC is considered to be a coordination catalyst, and thus study on LTMC has been limited to olefin-related polymerizations. No prior art discloses the use of LTMC for making hydroxyl acrylic resins, styrene-allyl alcohol copolymers, and many other important functional polymers. No prior act discloses the use of LTMC for the polymerization of unsaturated monomers in the absence of olefins.
Compared to conventional free radical polymerization, the LTMC has great potential in tailoring of critical polymer properties: molecular weight, crystallinity or melting point, and polydispersity. Therefore, the LTMC may provide better product quality and production consistency. Also, the LTMC does not require high temperature and high pressure polymerization. It avoids the use of explosive peroxides or azo compounds. Thus, the LTMC polymerization may provide a safer and more cost-effective alternative to the existing free radical technology.
In summary, it is apparently important to explore the use of LTMC for the polymerization of the unsaturated monomers which have been traditionally, some of which have been exclusively, polymerized by free radical polymerizations.